1. Field of the Invention
The present invention relates to an improvement in an apparatus for driving a solid-state scanning head (printer head) used as an exposure means in an image forming apparatus such as an electrophotographic printer, a digital copying machine, or a facsimile apparatus.
2. Description of the Related Art
In some conventional electrophotographic printer using an electrophotographic process, a printer head is used as an exposure means for forming an electrostatic latent image on a photosensitive member (image carrier) by scanning/exposing the photosensitive drum in accordance with image data to be printed.
As printer heads of this type, for example, the following heads are available: a head having an array of a large number of light-emitting elements such as LEDs (light-emitting diodes), EL (electroluminescent) elements, or phosphors and designed to selectively cause the light-emitting elements to emit light in accordance with image data; and a head having an array of a large number of optical shutters consisting of liquid crystal elements, PLZT (lead lanthanum zirconate titanate), and the like and designed to control light from a light source by selectively controlling the switching times of the optical shutters in accordance with image data.
Generally, in such a printer head constituted by a large number of light-emitting elements or optical shutters, the light-emitting elements vary in lighting intensity or the optical shutters vary in transmittance. If, therefore, gradation recording is performed by simply controlling the lighting times of the light-emitting elements or the switching times of the optical shutters of this printer head, the print result will suffer density irregularity caused by variations in lighting intensity of the light-emitting elements or variations in transmittance of the optical shutters.
For this reason, such a printer head is generally used only for a binary printer.
Recently, in a laser printer as a means for achieving high image quality, for example, the following operations have generally been performed: finely controlling the lighting time of a laser beam to perform gradation recording; and performing a smoothing process (round edge treatment: RET) to print an oblique line portion or a curved line portion smoothly.
In a printer using a printer head, however, the light timing or switching timing of each of a large number of light-emitting elements or optical shutters must be controlled. For this reason, it is very time-consuming to transfer print data to the printer head, resulting in a considerable drop in print speed of the printer.
In a method of expressing gradation by controlling the lighting time at each exposure point, a reduction in exposure area is inevitable. In this case, as the exposure area becomes smaller, developing of an electrostatic latent image becomes unstable, resulting in a deterioration in image quality of a print result.